Electric fuse for retarded or rapid operation



Sept. 18, 1956 PAETOW 2,763,752

ELECTRIC FUSE FOR RETARDED 0R RAPID OPERATION Filed Sept. 30, 1954 United States Patent ELECTRIC FUSE FOR RETARDED 0R RAPID OPERATION Heinz Paetow, Amberg, Germany, assignor to Siemens- Schuckertwerke Aktiengesellschaft, Berlin Siemensstadt and Erlangen, Germany, a German corporation Application September 30, 1954, Serial No. 459,276 Claims priority, application Germany December 4, 1953 11 Claims. (Cl. 200-435) My invention deals with electric fuses of the open and enclosed types as used for retarded protection against persistent overloads of moderate magnitude or for rapid interruption of short circuits or other overloads of greatly excessive magnitude. More particularly, my invention relates to fuses whose fuse conductors carry a trigger substance of selenium which, when its melting temperature is reached, aggressively reacts with the metal of the conductor thus causing the fuse to blow. Such seleniumtriggered fuses are disclosed in the copending application of Georg Iwantscheff and Walter Rauch for Retarded or Fast Acting Electric Fuses, Serial No. 459,277, filed September 30, 1954, and assigned to the assignee of the present invention.

A selenium-triggered fuse has the advantage of a relatively low steady-state operating temperature in conjunction with a fuse characteristic that is practically independent of changes in ambient room temperature. Since the selenium acts upon the fuse conductor only when melting and then so vigorously attacks the conductor that it is rapidly destroyed, a fuse with selenium-triggered conductors is virtually free of aging phenomena.

With respect to manufacturing requirements, however, the selenium-triggered fuses involve considerable dilficulties. When the fuse conductor of silver or copper is coated with selenium by dipping the conductor into a selenium melt, then the conductor metal is often appreciably attacked by the selenium even if the coating process is carried out with greatest rapidity, and the resulting changes in the fuse characteristic are not always alike so that it is extremely difficult to produce a series of fuses in mass production with accurate predetermined and uniform characteristics.

When the selenium is precipitated upon the fuse conductor by vaporization, a rather intricate manufacturing process is required because the vaporization of the selenium must be carried out in vacuum, and relatively thick selenium layers of several tenths of one millimeter must be formed so that the processing period is rather long. When applying the selenium as a powder by means of an adhesive, the chemical reaction, particularly in the temperature range immediately above the selenium melting point, is retarded and is not uniform throughout a series of fuses.

It is, therefore, an object of my invention to improve selenium-triggered fuses so as to eliminate the above mentioned disadvantages.

Another object of the invention is to improve such fuses as regards their applicability for time-lag response to overload conditions.

According to my invention the selenium is attached to the fuse conductor but is separately deposited upon a carrier body, and this carrier body is mounted on the fuse conductor to form a single fuse element together therewith. The carrier body, particularly for overload-responsive fuses of retarded time characteristic, consists preferably of a metal that, in contrast to the fuse conductor proper, is resistant to molten selenium. This not directly 2,763,752 Patented Sept. 18, 1956 carrier metal consists, for instance, of iron or aluminum. However, the carrier for the selenium may also consist of insulating material, particularly when the fuse is to have a rapid action.

The foregoing and other objects and features of the invention will be apparent from the following description of the embodiments illustrated on the drawings in which Figs. 1, 2 and 3 are perspective illustrations of three different fuse elements; Fig. 4 shows another fuse element With indirect heating means; and Fig. 5 is a cross section of an enclosed fuse also designed for control by indirect heating.

According to Fig. 1 a strip-shaped conductor 1 of a fuse element has two punched-out recesses 2 and 3 that form lateral shoulders. A sheet-metal part 4 coated with selenium is bent about the recessed portion of the fuse conductor so as to be prevented by the shoulders from becoming displaced. The sheet or strip structure 4 is coated with selenium before it is placed upon and bent about the fuse conductor. When bending the structure 4, it is preferable, though not absolutely necessary, to heat the structure 4 or also the fuse conductor, as well as the bending tool or jig used, to a somewhat elevated temperature.

According to a modification of the design shown in Fig. l, the structure 4 may consist of a selenium-coated wire or strand element which is wound about the fuse conductor. In this case the fuse conductor may be bent to a cylindrical shape surrounded by the coil of seleniumcoated wire.

The embodiments shown in Figs. 2 and 3 have the Ac- 5 coated with selenium 7 of the fuse conductor metal, coated with selenium, is oined with the fuse conductor 10 by means of two lateral lugs 11 and 12 formed by the fuse-conductor punching. After placing the selenium-coated structure 9 onto the fuse conductor 10, the lugs 11 and 12 are bent to the illustrated position, thus joining the carrier 9 with the conductor. The carrier 9 has lateral recesses so as to form shoulders which prevent the displacement of the carrier.

Fuse elements according to the invention, as exemplified by the above-described embodiments, afford a number of advantages. When depositing the selenium substance directly upon ing the ends of the terminals or parts of a housing. Such defects are avoided with fuse elements according to the invention. Furthermore, by properly selecting the size and mass of the selenium-carrying the thermal capacity of the fuse element at the critical place of the element can be modified within Wide limits, thus obtaining a corresponding modification of the operating characteristic and time lag of the fuse. For instance, when providing a relatively thick piece of sheet metal as the carrier of the selenium substance, the fuse can be given a large time lag, i. e. the period of time elapsing from the moment when the current reaches an overload value to be responded to until the moment when the melting point of selenium is reached, can be extended in such a manner. By thus using carrier structures of respectively different mass and heat capacity, it is also possible to use the same carrier punchings for the construction of retarded as well as rapid-action fuses.

The fuse conductors consist of the electrically good conductive metals conventional in fuses, preferably silver or copper. These metals are not attacked by selenium as long as the selenium is solid. However, when copper or silver get in touch with molten selenium, a very aggressive reaction takes place and results in the formation of silver selenide or copper selenide. Both compounds are virtually insulating so that the conductive cross section of the fuse conductor becomes constricted. As a result the fuse blows immediately as soon as the trigger conditions are reached.

The selenium-coated carri r body consists of a material not appreciably attacked by molten selenium. Metallic materials well suitable for the carrier structure are preferably iron or aluminum. Both of these metals can be coated with a layer of selenium by submerging them for a short interval of time in rectified selenium. The adhesion' of the selenium coating to the metal is improved when care is taken to give the metal a clean surface before dipping it into the melt.

The selenium substance in fuses according to the invention may consist only of selenium or it may contain some admixture, such as sulphur, for reducing the melting point of the substance. Many fuses are equipped with a multiplicity of parallel fuse conductors so that it Would be inconvenient if the small selenium-coated carrier pieces 'had to be separately mounted on the individual conductors. Embodiments of the type shown in Figs. 2 and 3, however, afford simultaneously attaching the carriers in a simple manner to many parallel fuse conductors with the aid of suitable fabricating tools or jigs. With a design according to Fig. 2, a long wire is preferably coated with selenium and is then stuck through the slits of a number of fuse-conductor strips placed side by side in a suitable holding device. Thereafter, the wire pieces between the parallel conductors are cut off to proper length by punching. With a design according to Fig. 3, a long metal strip is first provided with a number of rectangular slits extending parallel to the width of the strip. Then the strip is coated with selenium and is thereafter placed across a number of parallel fuse-conductor strips 10. Thereafter, in a punching press, the selenium coated strip is severed into individual pieces 9, and the lugs 11, 12 are bent over the carrier pieces preferably in the same press.

The invention can be reduced to practice in a variety of other ways. For instance, the carrier body for the selenium substance may consist of insulating material, for instance plastic, mica or threads of asbestos. Since such substances have an especially slight thermal capacity, their use permits the production of very rapid fuses. It will further be apparent that the carrier structure for the selenium maybe mounted in any other desired manner in the fuse or on the fuse conductor itself.

A fuse element according to the invention further affords the possibility of applying indirect heating so that the fuse can be caused to blow by extraneous control. For instance, whenproviding a carrier structure of metal as described with reference to Fig, 3, the carrier can be eated' simply by passing an auxiliary current therethrough. In this case, the connection of the carrier structure into the separate heating circuit may be effected by leads that extend out of the enclosure of the fuse.

The fuses shown in Figs. 4 and 5 incorporate the justmentioned features. According to Fig. 4 a fuse element is composed of a conductor 13 and a selenium-coated carrier 14' designed and mounted substantially as described with reference to Fig. 1. In addition, the fuse element comprises a heater coil 15, for instance, of wire coated with ceramic, vitreous or other heat insulating material. The ends 16 of the heater are to be attached to a current source which permits controlling the temperature of the fuse element so that the fuse can be made to interrupt the load circuit. This can be made dependent upon conditions other than the magnitude or persistency of the load current then flowing through the fuse conductor 13.

Fig. 5 shows an enclosed fuse of the cartridge type. A cylindrical housing 17, for instance of ceramic material,

is closed at both ends by ferrule-type terminals 18 and 19 between which a fuse conductor 20 is attached. Mounted on the fuse conductor is a small carrier structure 21 of metal, preferably iron or aluminum. The carrier structure is coated with a layer 22 of selenium and is connected to leads 23 that extend laterally out of the housing 17 for supplying a separately controllable heating current. The carrier piece 21 may be mounted on the conductor 21 in any suitable manner.

It will be obvious to those skilled in the art upon a study f this disclosure that my invention permits of various modifications and applications other than those specifically set forth and illustrated, without departing from the essential features of the invention as set forth in the claims annexed hereto.

I claim:

1. An electric fuse, comprising a fuse conductor of electrically good conductive metal, a trigger substance essentially of selenium for initiating destruction of said conductor when said substance is heated to melting temperature, a carrier body of material resistant to molten selenium, said substance being deposited on said carrier body, and said carrier body being mounted on said fuse conductor.

2. An electric fuse, comprising a fuse conductor of electrically good conductive metal, a trigger substance essentially of selenium for initiating destruction of said conductor when said substance is heated to melting temperature, a carrier body of metal resistant to molten selenium, said substance being deposited on said carrier body, and said carrier body being mounted on said fuse conductor.

3. In an electric fuse according to claim 2, said fuseconductor metal being selected from the group consisting of copper and silver, and said metal of said carrier body being selected from the group consisting of iron and aluminum.

4. In an electric fuse according to claim 1, said carrierbody consisting of insulating material.

5. An electric fuse, comprising a fuse conductor, and a carrier structure attached to said conductor and having a coating of selenium contactable, when molten, with said conductor.

6. In an electric fuse according to claim 1, said carrier body consisting of a strand element disposed about said conductor so as to be joined therewith, and said substance forming a coating on said strand element.

7. In an electric fuse according to claim 1, said con-- ductor being slitted, said can'ier body extending through the slits of said conductor so as to be joined therewith, and said substance forming a coating on said body.

8. In a fuse according to claim 1, said fuse conductor having lugs of bent shape engaging said carrier body to secure it to said conductor.

9. An electric fuse, comprising two mutually spaced load-current terminals, a fuse conductor interconnecting said terminals, a carrier body of material resistant to molten selenium, said body being mounted on said conductor in spaced relation to said two terminals and having a coat of trigger substance essentially of selenium to be active upon said conductor when said substance reaches melting temperature.

10. An electric fuse, comprising two mutually spaced load-current terminals, a fuse conductor interconnecting said terminals, a carrier body of material resistant to molten selenium, said body being mounted on said conductor in spaced relation to said two terminals and having a coat of trigger substance essentially of selenium to be active upon said conductor when said substance reaches melting temperature, and a heating device thermally joined with said carrier body for heating said body.

11. An electric fuse, comprising two mutually spaced load-current terminals, a fuse conductor interconnecting said terminals, a carrier body of metal resistant to molten sel nium sa d body being mounted on said conductor in 5 spaced relation to said two terminals and having a coat 1,447,646 Cherry Mar. 6, 1923 of trigger substance essentially of selenium to be active 1,660,828 Bird a Feb. 28, 1928 upon said conductor when said substance reaches melting 2,703,352 Kozacka Mar. 1, 1955 temperature, and a heating circuit having leads connected FOREIGN PATENTS with sand carner body for heatlng 1t lndependently of 5 current flowing through said terminals. 101,330 Sweden P 1941 473,335 Great Britain Oct. 7, 1937 References Cited in the file of this patent UNITED STATES PATENTS 1,375,474 Smelling Apr. 19, 1921 

